1. Field of the Invention
The present invention relates to a crystallized glass and a method of producing the same and also relates to a substrate for magnetic disc obtained by using the crystallized glass and a magnetic disc utilizing the substrate for magnetic disc.
2. Description of Related Art
In a field of magnetic disc substrate, a glass having a thermal expansion coefficient ((90.about.130).times.10.sup.-7 /K) near that of SUS (for example thermal expansion coefficient of 105.times.10.sup.-7 /K) used for fixing the disc is required due to a problem of thermal off track. In an amorphous glass such as chemical strengthened glass, it is necessary to increase an amount of alkali metal oxide so as to make a thermal expansion coefficient thereof larger. As a result, since a weather resistance of the glass is deteriorated, it is difficult to make a thermal expansion coefficient at near room temperature to (90.about.130).times.10.sup.-7 /K. In the crystallized glass, it is possible to make a thermal expansion coefficient at near room temperature to (90.about.130).times.10.sup.-7 /K by precipitating crystals having a larger thermal expansion coefficient.
Generally, a glass having a thermal expansion coefficient such as (90.about.130).times.10.sup.-7 /K at 25.degree. C..about.100.degree. C. was required. However, recently a glass having a thermal expansion coefficient such as (90.about.130).times.10.sup.-7 /K at lower temperature range of -50.degree. C..about.70.degree. C. is required.
In Japanese Patent Publication No.6-29152 (JP-B-6-29152), a glass containing P.sub.2 O.sub.5 and MgO as an essential ingredient, which has .alpha.-cristobalite and lithium disilicate as a main crystal and .alpha.-quartz and lithium metasilicate as a sub crystal, is disclosed. Since, in the glass mentioned above, cristobalite phase is precipitated as a main crystal, a variation of thermal expansion coefficient with respect to a crystallization temperature is low, but a particle size of crystal is large such as smaller than 1 .mu.m. In Japanese Patent Nos. 2516553 and 2628460, a crystallized glass for magnetic disc having .alpha.-quartz and lithium disilicate as a main crystal, in which .alpha.-quartz is precipitated as a coagulation state having a diameter of 0.3 .mu.m or more, is disclosed. In these disclosures, a stability of thermal expansion coefficient with respect to the crystallized temperature is not mentioned. Moreover, since .alpha.-quartz is precipitated as a coagulation state, a surface roughness (Ra) after a precise polishing is large such as 5.about.15 angstroms.
In Japanese Patent Laid-Open Publication No. 10-226532 (JP-A-10-226532), a crystallized glass for magnetic disc having lithium disilicate as a main crystal and .alpha.-quartz as a sub crystal is disclosed. In this disclosure, it is disclosed an example that a surface roughness after a precise polishing is 6.5 angstroms, but a particle size of crystal and a stability of thermal expansion coefficient with respect to the crystallized temperature are not mentioned. In Japanese Patent Laid-Open Publication No. 7-101750, a crystallized glass containing P.sub.2 O.sub.5 and TiO.sub.2 as an essential ingredient, which has .alpha.-quartz and lithium disilicate as a main crystal, is disclosed. In this disclosure, it is described that a stability of thermal expansion coefficient with respect to the crystallized temperature is excellent, but it is not preferred for a magnetic disc due to its worse weather resistivity since there is a lithium metasilicate phase.
In Japanese Patent Laid-Open Publication No. 10-208226 (JP-A-10-208226), a crystallized glass containing Nb.sub.2 O.sub.5, which has lithium disilicate and (.alpha.-quartz as a main crystal, is disclosed. In this disclosure, an object of including Nb.sub.2 O.sub.5 is to improve a light absorption when performing a laser texture working, and there is no description about a thermal expansion coefficient of the crystallized glass and a stability of thermal expansion coefficient. Moreover, it is disclosed that an amount of Nb.sub.2 O.sub.5 is preferred to 0.01.about.0.5 wt %. In Japanese Patent Laid-Open Publication No.7-157331 (JP-A-7-157331), a crystallized glass containing molybdenum oxide and so on and showing Mansell value of 0.about.7, which has lithium disilicate and .alpha.-quartz as a main crystal, is disclosed. In this disclosure, an object of including Mo and so on is to easily detect surface flaws. In this invention, it is possible to add molybdenum oxide up to 4 wt %, but, in this case, even if combining Nb.sub.2 O.sub.5 and SnO.sub.2, Mansell value showing a brihgtness is not smaller than 7.